Method of producing fluidized material from a subterranean formation



y 15, 1969 M. PRATS ET'AL 3,455,383

METHOD OF PRODUCING FLUIDIZED MATERIAL FROM A SUBTERRANEAN FORMATION Filed April 24, 1968 INVENTORS:

M. PRATS P. VAN MEURS 7 BY: W

THEIR ATTORNEY United States Patent 3,455,383 Patented July 15, 1969 3,455,383 METHOD OF PRODUCING FLUIDIZED MATERIAL FROM A SUBTERRANEAN FORMATION Michael Prats, Houston, Tex., and Pieter Van Meurs, New Orleans, La., assignors to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Apr. 24, 1968, Ser. No. 723,735 Int. Cl. E21]! 43/24, 47/04 US. Cl. 166-254 12 Claims ABSTRACT OF THE DISCLOSURE A method of producing fluidized material from a normally impermeable fluidizable .material-bearing subterranean earth formation wherein a plurality of wells extending into the earth formation are interconnected by a generally horizontally extensive channel located adjacent substantially the bottom of the formation. The channel is held open by circulating heated fluid threthrough at a pressure sufficient to lift the roof of the channel. The degree to which the earth formations overlying the channel are lifted is monitored as the heated fluid is being circulated and fluidized material is being produced. At a point when these overlying formations are lifted a selected distance, the circulation of heated :fluid is stopped and the roof of the channel is lowered under the weight of the overlying formations thereby crushing and compacting partially depleted material within thechannel. The production of fluidized material is resumed by again circulating heated fluid through the channel at a pressure suflicient to lift its roof. The channel migrates upwardly as depleted material is released from the roof.

BACKGROUND OF THE INVENTION A Field of the invention channel interconnecting a plurality of wells to produce a maximum amount of fluidized material with a minimum amount of lifting of the earth formations overlying the channel.

Description of the prior art Oil shales which occur, for example, in the United States contain amounts of potentially. recoverable hydrocarbons many times greater than the known petroleum reserves in the continental United States. Kerogen-impregnated fresh water marls, such as the Green River a formation, constitute one of the greatest single noncoal deposits of potentially liquid hydrocarbon in the world. Many attempts and known prior proposals have been directed to producing salable petroleum commodities by in situ recovery of shale oil from theoil shale. However, none of the known prior proposals have been commercially successful. n

There are a number of problems involved in the in situ recovery of the petroleum products of oil shale which contribute to the failure of the prior attempts at in situ recovery. These problems are created mainlyiby the nature of the oil shale, which isa kerogen-impregnated material that is dense, practically impervious tofluids, and con- .tains only about one percent moisture. Further, formations of oil shale, although dense and impervious; conopenings through which fluid can flow. In addition, when fluid that is heated to an oil shale pyrolyzing temperature is circulated through a channel within a subterranean oil shale, the oil shale tends to become heated and expanded before fluid released by pyrolysis can flow out of the heated portions. If the channel is vertical, it tends to become plugged by the heat-induced expanding or swelling of the channel walls. If the channel is horizontal, its roof can be held up by the pressure of the circulating fluid; but, since organic material within the roof tends to expand more rapidly than pyrolysis products can flow into the channel, chunks or fragments of oil shale are released by the exfoliation or spalling-off of the roof material and are displaced into the channel.

In general, the fragments that spall off the roof of such a channel are mechanically strong, impermeable materials having surfaces from which most of the organic material has been removed. In a horizontal channel, a layer of such fragments of partially depleted oil shale is permeable to fluid flowing around, but not through, the individual fragments. As such a layer builds up, it occupies space into which new fragments from the oil shale roof would otherwise be displaced and thus it impedes the release and displacement of new fragments and impedes the development of undepleted surfaces of oil shale that can be contacted by the circulating heated fluid.

In the past, it has been proposed that oil be produced from a subterranean oil formation by circulating heated fluid through a generally horizontal channel at a pressure suflicient to lift the roof of the channel. Prior processes of this type are described in US. Patents Nos. 2,969,226 and 3,284,281. The thickness of such channels increases as fragments of partially depleted oil shale spall off the channel roof and the roof is lifted above the bottom of the channel in order to accommodate the increase in porosity of the oil shale formation. This is accomplished by increasing the volume of fluid in the channel and bending and/or fracturing the overlying earth formations as the channel roof migrates up through the oil shale formation. The channel roof can be lifted and held at the selected height by circulating fluid through the channel at a selected rate and pressure. This roof-lifting can be done before or after the spalling-off of sufficient oil shale material 'to fill the channel with partially depleted oil shale. If the circulation of heated fluid is continued either while maintaining the channel roof at a given height above the bottom of the channel or while migrating the channel roof up to the same height, there is a decline in the rate at which oil is produced from the layer of oil shale having a thickness corresponding to the distance between the bottom and the roof of the channel. If the channel roof is first lifted over the total distance, an increase occurs in the proportion in which oil is contained in the outflowing =fluid, but this is followed by a later, relatively rapid, decline in the rate of oil production. On the other hand, if the channel roof is migrated upward while oil is being produced, there is a continual decrease in both the amount of spalledoff oil shale material and the rate of shale oil production. In either case, the amount of shale oil production is limited to the amount that can be released by pyro- -lyzing a virgin oil shale and the thickness of the porous region, and the tensile strength, brittleness, and the like properties of the earth-formations overlying the channel roof. In general, it is important to minimize the extent to which the overlying earth formations must be bent.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved method of producing fluidized material from a normally impermeable fluidizable material-bearing subterranean earth formation that remains impermeable when the fluidizable material is removed whereby heated fluid is circulated through a generally horizontal channel in the formation at a pressure sufficient to lift the roof of the channel.

It is a further object of this invention to provide a method of producing a maximum amount of fluidized material from such formation by lifting the earth formations overlying a generally horizontal channel extending through the formation a selected amount insufficient to extend fractures to the surface of the earth.

It is another object of this invention to provide a method for economically compacting oil shale material left in a generally horizontal channel extending through an earth formation to a minimum volume of densely packed particles from which volume-occupying and intergranular cementing organic components have been removed.

The above and other objects of this invention are carried out by circulating heated fluid at a pressure sufficient to lift the roof of a generally horizontally extensive channel interconnecting a plurality of wells extending into an earth formation, the channel being adjacent the bottom of the normally impermeable fluidizable material-bearing subterranean formation. The degree to which the earth formations overlying the channel are lifted is monitored, preferably While the fluid is being circulated and fluidized material is being produced. The monitoring can be continuous or intermittent. At a point when these overlying formations are lifted a selected distance, the circulation of heated fluid is stopped, or at least interrupted by a significant curtailment of circulation rate, while the roof of the channel is lowered under the weight of the overlying formations, thereby crushing and compacting partially depleted material within the channel. The production of fluidized material is resumed by again circulating heated fluid through the channel at a pressure sufficient to lift its roof. The channel migrates upwardly as depleted material is released from its roof.

Although the teachings of this invention are applicable to any normally impermeable fluidizable material-bearing subterranean earth formation that tends to remain impermeable when the fluidizable material is removed, the present invention is particularly useful in producing shale oil from a subterranean oil shale. The method of this invention increases the amount of broken and spalled oil shale which can be accommodated when earth formations, overlying the roof of a channel through which a heated fluid is circulated, are lifted only a given distance above the original channel floor. For example, if it is desired to raise the overburden no more than ten feet, if the spalled material is allowed to accumulate within the channel until it contacts the roof of the channel, and if the porosity of the random pile of spalled material is 0.4, then the maximum vertical interval of oil shale which can be broken and spalled is 25 feet. However, if the lowering of the overburden is used to recompact the broken and spalled spent oil shale within the channel to an average porosity of 0.1, then the vertical interval of oil shale which can be broken and spalled becomes 100 feet.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is a schematic illustration of a plurality of wells extending into a subterranean oil shale formation from which shale oil is to be produced; and

FIGURES 2 and 3 are diagrammatic illustrations of the carrying out of the concepts of the invention.

4 DESCRIPTION OF THE PREFERRED EMBODIMENT Although this application will be described with particular reference to a subterranean oil shale formation, the teachings of this invention are applicable to other normally impermeable fluidizable material-bearing subterranean earth formations.

In forming a generally horizontal channel of fluid communication between a plurality of wells opening into a subterranean earth formation, it is preferable to form a channel in which the permeability is due mainly to the existence of a horizontal fracture that interconnects he wells. Alternatively, or additionally, the channel can be formed by utilizing substantially any combination of natural fractures, streaks or porous materials, explosively or hydraulically induced fractures, etc., which intersect to form a generally horizontal channel of fluid communication.

The patterns of these Wells and the sizes of these patterns are not critical except that, for example, if the oil shale is relatively deep and the Well pattern is relatively small, the expense and risk of lifting the overlying earth formations high enough to permit the recovery of an economically attractive amount of oil may be significant. The well pattern may be completed before, during, or after the initiation or completion of the drilling and fracturing operations by which a generally horizontal channel of fluid communication is extended along the areal extent of a well pattern.

The regional tectonics in locations in which subterranean oil formations are encountered, particularly in respect to oil shale formations of relatively shallow depth, such as less than about 6,000 feet, are likely to create a tendency for fractures to extend along generally vertical planes through the oil shale formation and the earth formations overlying the oil shale formation.

The present invention is preferably accomplished by thermally biasing an oil shale formation to cause fractures to extend horizontally and extending a generally horizontal fracture into fluid communication with wells which form a pattern having an average diameter equal to at least approximately one-half of the depth of the oil shale formation. Such a thermal biasing is preferably accomplished by treating one or more wells by any known prior art process, such as the process disclosed in an application Ser. No. 578,533, to Matthews et al., filed on Sept. 12, 1966. In this application, hot liquid is injected into an oil shale formation through natural fractures if they exist or fractures which are induced hydraulically by pressurizing the hot liquid being injected into the wells while using liquid heating and liquid injection rates that cause heat to be transported for significant distances into the oil shale formation. Any vertical fractures which are formed, or were pre-existing, become plugged as the oil shale along their walls becomes heated and expanded. Since stresses build up due to the horizontal expansion, as the expanding portions push horizontally against the surrounding portion of the oil shale and other earth formations, the pressure necessary to maintain a selected rate of liquid injection increases and builds up until it reaches a pressure suflicient to lift the roof of a horizontal fracture and to overcome the vertical tensile strength of the oil shale formation. .This pressure induces a horizontal fracture. Such a horizontal fracture is pref-' erably kept open, extended throughout a pattern of wells, and heated to a temperature which provides a suitable rate of oil shale pyrolysis. This is preferably carried out by increasing the temperature at which heated fluid is pumped through the fracture at a rate such that a temperature sufiicient to pyrolyze the kerogen is obtained while using a pumping pressure not significantly greater than the fracture-roof-lifting pressure, for example by the procedure described in an application Ser. No. 578,244 to Matthews et al., filed on Sept. 9, 1966, now Patent No. 3,379,250.

Various prior art processes and apparatus arrangements for producing oil by circulating heated fluid through a generally horizontally extensive channel of communication between a pattern of wells can be utilized in practicing the present invention. Suitable fluids include steam, aqueous liquids, gases, vaporous or liquid hydrocarbons, combinations of such fluids, etc. Suitable apparatus may include conventional apparatus for pumping, heating, produced-fluidthrottling, separating, oil-recovering and fluid recycling, etc.

When desirable, the heated fluid being circulated through such a channel may be heated downhole by flowing the fluid into heat exchanging relationship with one or more downhole heaters, such as electric heaters, gasfuel heaters and/or downhole nuclear reactors, or the like.

In accordance with the teachings of this invention, after a generally horizontal channel is extended along at least a portion of a well pattern, the production of oil is initiated by circulating a heated fluid through the channel as discussed hereinabove and the extent to which the overlying earth formations are lifted is monitored. This monitoring can be accomplished by a variety of techniques. For example, such techniques may include measuring the rise of a near-surface location or structure above a point of reference, measuring the volume of the circulating fluid that is present in the channel as indicated by the difference between the amounts injected and produced, measuring the rate at which the roof of such a channel of fluid communication migrates through a sample or model oil shale earth formation during an equivalent oil production procedure, etc. It is generally preferable to utilize a combination of measurements such as the amount of the surface elevation and the amount of increase in the volume of fluid in the channel.

Referring now to FIGURE 1, a series of earth formations represented by earth formation 1 is shown overlying a subterranean oil shale formation 2. A plurality of wells 3, comprising a pattern 4, extends through earth formation 1 into communication with oil shale formation 2. A generally horizontal channel 5 is formed adjacent substantially the bottom of the oil shale formation 2 in a manner discussed hereinabove. For example, as illutrated in FIGURE 1, where the oil shale is located at a depth such as 2,000 feet, a preferred pattern 4 may comprise a square having sides along which there are nine wells spaced approximately 125 feet apart thereby forming a pattern that is made up of a series of five-spot injection-production well groups and has a diameter of approximately 1,000 feet. The oil shale formation 2 is shown as having a thickness of approximately 125 feet and one of the buildings, trees, etc., disposed on the surface 6 of earth formation 1 may have an approximate height of 50 feet as a point of reference for illustrating the rise of the earth formations overlying channel 5. The channel 5 preferably extends through oil shale formation 2 along an area having an average diameter equal to at least substantially one-half the distance from the surface 6 to the top of the oil shale formation 2. When heated fluid is circulated into and through channel 5 at a pressure sufficient to lift the roof thereof in the manner discussed hereinabove, the amount by which the earth formations overlying channel 5 are lifted may become significant, as shown in FIGURE 1. For example, FIGURE 1 shows the fracturing to the surface with its necessary hazards to personnel and equipment that could result from an extensive lifting of the roof of channel 5 to produce all of the oil in place in formation 2. Alternatively, where economic factors, such as lease dimensions or permissible investments in materials and equipment, coupled with the properties of the subsurface earth formations, limit the extent to which it is desirable to lift the overlying earth formations, the thickness of the layer from which oil can be produced is limited. For example, where the lifting of the overlying earth formations is confined to 10 feet for various reasons and the porosity of the partially depleted oil shale materials that spalls ofl the roof of channel 5 into channel 5 amounts to 0.4 of the volume of the material, the thickness of the layer from which oil can be produced is limited to 25 feet. Since the exemplary oil shale formation 2 of FIGURE 1 has a thickness of 125 feet, the amount of shale oil which can be produced therefrom is drastically limited and is only one-fifth of the oil in place.

In accordance with the teachings of this invention, however, the oil-producing circulation of heated fluid is periodically interrupted when the degree to which the overlying formations are lifted reaches a selected, such as from a few inches to a few feet, amount, and the roof of channel '5 is periodically lowered to crush and compact the partially depleted oil shale in channel 5. As schematically illustrated in FIGURES 2 and 3, when the maximum lifting of the surface is confined to 10 feet, which can be determined by any of the aforementioned methods such as by reference to a known point of elevation, or a calculation based on the amount of fluid in channel 5, the crushing and compacting of the partially depleted oil shale 7 (FIGURE 2), having a relatively high porosity, such as 0.4, and a thickness of 25 feet, converts it to a compacted rubble 8 having a relatively low porosity, such as 0.1, the thickness of the layer of oil shale from which shale oil can be produced having increased to feet. Where the well pattern is relatively large and the overlying formations are adequately elastic, this avoids the danger of extending fractures to the surface as shown in FIGURE 1. The final rubbled zone 9 in FIGURE 3 illustrates how the teachings of this invention may be used where, in a given situation, only a given degree of lifting of the overlying earth formations is permitted; thus the amount of oil that can be produced from a subterranean oil-bearing formation is increased, for example, by an amount such as four times more oil.

In periodically lowering the roof of the channel 5 in accordance with the process of the present invention, the circulation of the heated fluid is preferably interrupted by producing fluid from all wells communicating with the channel 5. This forces the channel roof to sink as a substantially rigid piston that is driven downward by the total weight of the overburden (i.e., the overlying earth formations). As indicated in FIGURE 2, the distance over which such a channel roof may be lowered and the amount of fluid that is produced from the channel 5 is apt to be significant. Where, for example, a onefoot thick channel filled with partially depleted oil shale having a porosity of 0.4 extends along a 1,000 foot square well pattern, as shown in FIGURE 1, if the roof of the channel 5 is lowered while the channel 5 is filled with a substantially incompressible fluid, the amount of fluid which is produced is about 65,000 barrels, or about one weeks production with 100 wells producing 100 barrels per day.

The periodic lowering of the roof for the channel of fluid communication is beneficial in both allowing more shale oil to be produced in a given situation, by compacting the partially depleted oil shale to a relatively small volume requiring a relatively small lifting of the overlying earth formations, and in increasing the rate at which shale oil is released from the partially depleted oil shale in the channel of fluid communication. This is accomplished by crushing spalled-oif fragments and mechanically fracturing partially released oil fragments which are still connected to the roof of the channel to expose new oil containing surfaces to contact with the circulating heated fluid. The subsidence of the channel roof applies a vertical stress that becomes very high on any chunk of partially depleted oil shale that is larger than the surrounding chunks. For example, at a depth of about 2,000 feet, where the roof sinks under an overburden pressure of about 2,000 p.s.i., if a large chunk presents a bearing surface of about one square inch that extends above the other chunks within about one square foot, the protruding bearing surface may be subjected to a pressure exceeding about 200,000 p.s.i. This crushing action also grinds or fractures partially depleted surfaces along the channel roof and, in addition, compresses the layer of crushed and ground, partially depleted oil shale materials into a relatively small volume having small spaces between the particles.

In general, the present process is applicable to producing substantially any fluidizable material from an impermeable subterranean earth formation which, when heated, forms or releases a fluid or fluidizable material while being exfoliated into surface-depleted fragments which remain mechanically strong and impermeable. The present invention reduces the extent of growth of a layer of such surface-depleted fragments. This reduces the amount by which the overlying earth formations must be lifted to accommodate a channel through which fluid can be circulated and thus increases the amount of fluidizable material that can be recovered from any given situation in Which the extent to which the overlying earth formations can be bent above the channel of fluid communication amounts to a limiting factor.

In preferred oil shale production applications of the present invention, the extent of growth of a layer of partially depleted oil shale fragments is further reduced by periodically initiating an in situ combustion reaction and advancing a combustion front through the layer of partially depleted oil shale fragments to fluidize and/or dissolve additional components of the depleted oil shale material. The volume-reducing effects of intermittently crushing the spalled-off fragments of partially depleted oil shale is preferably supplemented by removing portions of the material constituting such fragments.

In a preferred embodiment, this is accomplished by initiating an underground combustion of the organic carbonaceous residue that has been left by the pyrolysis reaction along the surfaces of the spalled-ofl fragments of oil shale and advancing the combustion front through the mass of spalled-ofl fragments. Such an underground combustion can be conducted by circulating an oxygencontaining, combustion-supporting gas through the channel. This can be done at a pressure sufficient to support the roof, before or after the roof subsidence operation. However, such a combustion is preferably conducted after the roof of the channel has been lowered onto the layer of fragments, with the combustion-supporting gas being circulated at a pressure just sufiflcient to maintain an adequate flow rate of combustion-supporting gas through the channel. Such a combustion is preferably conducted so that the combustion front temperatures exceed about 1,000 F. At such a temperature, in addition to consuming the organic carbonaceous residue, the combination causes chemical reactions among the inorganic components such as carbonates, sulfates, and the like in the oil shale fragments. Such inorganic reactions convert solid components to fluids, such as carbon dioxide, sulfur trioxide, and the like, which are removed by the fluid circulation through the channel. In the presence of water, and water is formed by such an underground combustion, acid anhydrides such as CO S etc., form aqueous acids that dissolve additional portions of the depleted oil shale materials. Although the combustion may materially reduce the volume occupied by the components of the depleted oil shale materials, it does not, by itself, produce much oil. The partially depleted spalled-off fragments of oil shale contain significant proportions of combustible carbonaceous residue and thus, the bulk of the material to which such an in situ combustion process is applied has already been essentially depleted of producible hydrocarbons. The hot combustion products are like any heated fluid being circulated through the rubble-filled channel in respect to finding relatively few fresh, oil-containing surfaces from which oil can be released. The rate of oil production remains relatively low until the roof of the channel is hydraulically lifted above the layer of depleted oil shale materials so that new fragments can spall-otf to expose new surfaces from which oil can be extracted. The inorganic material conversions that accompany a high temperature underground combustion reaction provide the added advantage of materially reducing the mechanical strength of the partially depleted oil shale fragments and tend to weaken these fragments. Allowing the channel roof to settle onto the fragments that have been so heated causes the fragments to be more completely crushed to relatively very fine particles that become compacted into a relatively very small volume.

The mineral-component-dissolving effects of the acids formed by an in situ combustion reaction can be supplemented, or replaced, by circulating an acid through the channel. Such a solution mining or acidization of the fragments of partially depleted oil shale can be accomplished by conventional means such as well acidizing or solution mining methods and materials. For example, an aqueous 15 percent solution of hydrochloric acid can be circulated through the channel at a pressure less than or greater than a pressure sufficient to lift the roof of the channel.

Shale oil is then recovered from the shale oil-bearing fluid extracted from the channel 5 by means well known in the art, such as, for example, by passing the shale oilbearing fluid through a heat exchanger and into a separator where oil and gas components are separated as disclosed in an application Ser. No. 662,110 to Closmann, filed Aug. 21, 1967.

The preceding description of the invention is intended to be merely explanatory. Changes in the details of the described equipment and procedures may be .made, within the scope of the appended claims, without departing from the spirit of the invention.

We claim as our invention:

1. A method of producing a fluidized material from a normally impermeable fluidizable material-bearing subterranean earth formation wherein a plurality of wells extending into communication with said earth formation are interconnected by a generally horizontally extensive channel in said formation, said channel having a roof which is located adjacent substantially the bottom of said earth formation, the method comprising the steps of:

holding said channel open by circulating heated fluid thereinto from at least a first well at a pressure sufficient to lift the roof of said channel; monitoring the degree to which the earth formations overlying said channel are lifted as said heated fluid is circulated and fluidized material is produced from said channel through at least a second well;

detecting when said overlying earth formations have been lifted a selected distance insuflicient to extend fractures to the surface of the earth;

interrupting the circulation of said heated fluid by producing fluidized material-bearing fluid from said channel through at least said second well without injecting an equal amount of heated fluid into said channel through at least said first well;

lowering the roof of said channel under the weight of said earth formations overlying said channel thereby crushing and compacting any material partially depleted by said circulation of fluid through said channel; and

resuming, the production of fluidized material by circulating heated fluid through said channel at a pressure suflicient to lift the roof of said channel thereby causing said channel to migrate upwardly within said earth formation as depleted material is released from the roof of said channel.

2. The method of claim 1 including the steps of repeating the steps of continually raising and lowering the roof of said channel as frequently as necessary in order to produced fluidized material while confining, within a se lected limit, the degree of lifting of said earth formations overlying said channel.

3. A method of producing shale oil from a subterranean oil shale formation wherein a plurality of wells extend into communication with said oil shale formation; the method comprising the steps of:

interconnecting at least a portion of said wells by a generally horizontal extensive channel, said channel having a roof and being located adjacent substantially the bottom of said oil shale formation;

holding said channel open by circulating heated fluid thereinto from at least a first well at a pressure sufficient to lift the roof of said channel;

monitoring the degree to which the earth formations overlying said channel are lifted as said heated fluid is produced from said channel through at least a second well; detecting when said overlying earth formations have been lifted a selected distance insufficient to extend fractures to the surface of said earth formation;

interrupting the circulation of said heated fluid by producing shale oil-bearing fluid from said channel through at least said second well without injecting an equal amount of heated fluid into said channel through at least said first well;

lowering the roof of said channel under the weight of said earth formations thereby crushing and compacting any oil shale material partially depleted by said circulation of fluid through said channel; and resuming the production of shale oil-bearing fluid by circulating heated fluid through said channel at a pressure sufiicient to again lift the roof of said channel thereby causing said channel to migrate upwardly within said earth formation as depleted oil shale material is released from the roof of said channel.

4. The method of claim 3 including the steps of repeating the steps of continually raising and lowering the roof of said channel as frequently as necessary in order to produce shale oil-bearing fluid while confining, within a selected limit, the degree of lifting of said earth formations overlying said channel.

5. The method of claim 3 wherein the step of interconnecting at least a portion of said wells by a generally horizontally extensive channel includes interconnecting said wells by a channel which extends through the subterranean oil shale formation along an area having an average diameter equal to at least substantially one-half the distance from the surface of the earth to the top of the oil shale formation.

6. The method of claim 3 wherein the step of lowering the roof of said channel includes the stop of lowering said roof by producing shale oil-bearing fluid from substantially all of said wells communicating with said channel at substantially equal production rates.

7. The method of claim 3 including the step of recovering shale oil from said shale oil-bearing fluid.

-8. The method of claim 3 including the steps of: initiating underground combustion in the partially depleted oil shale material within said channel; and

advancing a combustion front produced by said underground combustion upwardly within said channel so as to fiuidize said partially depleted oil shale material and produce inorganic components from said partially depleted oil shale material.

9. The method of claim 8 wherein the step of initiating underground combustion is initiated after the roof of said channel has been lowered; and

said combustion front is advanced by circulating a combustion-supporting gas through said channel at a pressure just sufficient to maintain an adequate flow rate of combustion-supporting gas through said channel.

10. The method of claim 9 wherein the step of circulating said combustion-supporting as includes the step of circulating said combustion-supporting gas at a temperature high enough to maintain the combustion front temperatures above 1,000 F.

11. The method of claim 8 including the step of circulating an acid through said channel which is sufiicient to acidize the partially depleted oil shale material therein at a pressure below a pressure suflicient to lift the roof of said channel.

12. The method of claim 3 wherein the step of monitoring the degree to which the earth formations are lifted includes the step of measuring both the increase in the elevation of earth formations surrounding the wells and the increase in volume of fluid circulating through said channel.

References Cited UNITED STATES PATENTS 2,952,450 9/1960 P-urre 2994 3,316,020 4/1967 Bergstrom 16611 X 3,396,791 8/1968 Van Meurs et a1 166l1 CHARLES E. OCONNELL, Primary Examiner J. A. CALVERT, Assistant Examiner US. (:1. X.R. 166-272; 299-4 

